Since the fuel rod placed in the reactor core of a light water nuclear reactor is compromised of a Zircaloy cladding tube filled with uranium fuel pellets, once the cladding breaks, the nuclear fuel therein could be released into the reactor as to radioactively pollute the cooling water that, as a consequence, the safety of the whole nuclear power plant is affected and the radioactivity of the material discharged therefrom is increased. In a worse scenario, the operation of the nuclear power plant will have to be shut down for inspection so that causes a severe economic loss. In this regard, a method capable of detecting precisely the defects of a cladding tube can play an important role for preventing the above-mentioned economic loss by avoiding the foregoing radioactive pollution from happening. However, it is not an easy and convenient job for even a trained technician to identify a defect using the current ultrasonic inspection method that is usually the cause of detainment for a scheduled maintenance of nuclear power plant. Therefore, an improve inspection method, which is fast and precise, can save the maintenance cost in millions by the saving of working hours for the maintenance. It is one of the most important safety features for a nuclear power plant to be able to rapidly and precisely detect a fuel rod with defected cladding so as to proceed with the replacement of the defected fuel rod.
Refer to FIG. 1A and FIG. 1B, which are schematic illustrations showing a conventional ultrasonic inspection method used by most nuclear power plant for detecting defects of a fuel rod. As seen in FIG. 1A, an ultrasonic signal 111 emitted from an ultrasonic emitter 11 is vertically-incident to the cladding tube 12 of a fuel rod to be inspected that the ultrasonic signal 111 will travel through the internal 121 of the tube and finally to be received by the receiver 13. The magnitude of the ultrasonic signal 111 received by the receiver 13 will increase when there is cooling water accumulated in the internal 121 of the tube caused by a cracking of the cladding 12 enabling the cooling water to enter therefrom. By which, the situation of water accumulated inside a cladding tube can be detected.
FIG. 1B shows another method for detecting the situation of water accumulated inside a cladding tube. Similarly, an ultrasonic signal 111 emitted from an ultrasonic emitter 11 is vertically incident to the cladding tube 12 of a fuel rod and further into the internal 121 of the tube. If there exists the situation of water accumulated inside a cladding tube caused by the cracked cladding tube 12, the ultrasonic signal 111 traveling inside the tube will be attenuated and reflected and the reflected signal will be received by the receiver 11′. That is, if an attenuated ultrasonic signal is detected by the receiver 11′, there is surely water accumulated inside the cladding tube.
From the above description, the conventional inspection method has the following shortcomings:                (1) The reliability of the conventional inspection method is low, since a minute decrease of signal might not be caused by accumulated water resulting from defected cladding. Moreover, it is difficult to identify a variation in magnitude of the ultrasonic signal, since noise will have an effect while evaluating the magnitude of the signal.        (2) The convention inspection method can only determine whether there is water accumulated inside the cladding tube, but can not detect the exact position of cracking or defect.        (3) The inspection accuracy of the convention inspection method is easily affected by the shape of the object to be inspected. It is more suitable to be used for inspecting a flat object and not for a hollow tube or an object with curved surface.        (4) The convention inspection method is substantially a method of two-dimensional measuring, which is not as effective while it is used for inspecting defects of an upright cladding tube.Thus, it is indeed a pressing requirement for improving the conventional inspection method.        